The present invention relates to high-voltage bushings. More specifically, the present invention is concerned with a high-voltage bell bushing provided with external and internal shields used for switchgears, disconnect switches and other high-voltage related equipments.
The use of high-voltage bushings to surround a conductor which extends through a metal plate is well known in the art. Since such bushings are primarily used to insulate the conductor from the conductive wall through which it extends, they are made of a dielectric material.
It is often desirable to position a current sensor, usually in the form of a current transformer, near the bushing to monitor the current flowing through the conductor.
FIG. 1 of the appended drawings schematically illustrates, in a partly sectional view, a conventional high-voltage bushing 10 mounted to the conductive wall 12 of an enclosure. As schematically illustrated by electric field lines 14 in this figure, the electric field is intense at the base of the bushing, where the current transformer 16 is mounted. Since this strong electric field would interfere with the operation of a conventional current transformer, a shielded current transformer must be used, which increases the overall cost of the bushing.
It has been found that a shield mounted directly to the bushing and connected to the metallic enclosure to which the bushing is mounted allows a conventional current transformer to be used without interference from the electric field that is deflected by the shield. The shields are often molded with the bushing near an external surface thereof.
FIG. 2 of the appended drawings, which is labelled xe2x80x9cPrior Artxe2x80x9d, schematically illustrates a bushing 20 provided with an external shield 22 connected to the metallic wall 24 of the enclosure via the fastener 26.
As schematically illustrated by electric field lines 28 in this figure, the electric field is deflected from the area near the base of the bushing 20, thereby allowing a conventional current transformer 30 to be used.
It is to be noted that even though the shield 22 is schematically illustrated as a braided shield, it could advantageously be a capacitive shield made of a non-magnetic conductor material.
A major disadvantage of the bushing 20 is that the level of the electric field is very high in the air surrounding the conductor 34 which leads to high partial discharge levels in this area.
Another drawback of the bushing 20 is that the electric field goes through two dielectric material, i.e. the material forming the body 32 of the bushing and the air, having different dielectric properties which contributes to high partial discharges in the vicinity of the conductor.
An object of the present invention is therefore to provide an improved high-voltage bushing.
Another object of the invention is to provide a high-voltage bushing having both internal and external shield.
More specifically, in accordance with an embodiment of the present invention, there is provided a high-voltage bushing for insulating a conductor going through a metallic wall, the bushing comprising:
a generally tubular body made of a dielectric material; the generally tubular body being configured and sized to be mounted to the metallic wall and to receive the conductor; the generally tubular body having an external surface and an internal surface;
a generally tubular external shield embedded in the body near the external surface; the external shield being configured to be electrically connected to the metallic wall;
a generally tubular internal shield embedded in the body near the internal surface; the internal shield being configured to be electrically connected to the conductor.
According to a preferred embodiment of the present invention, there is provided a high-voltage bushing for insulating a conductor going through a metallic wall, the bushing comprising:
a generally tubular body made of a dielectric material; the generally tubular body being configured and sized to be mounted to the metallic wall and to receive the conductor; the generally tubular body having an external surface and an internal surface;
a generally circular flange made of dielectric material and integrally formed with the generally tubular body;
an outer insulating shell made of dielectric material and integrally formed with the flange; the outer insulating shell defining a series of skirts;
a generally tubular external braided metallic shield embedded in the body near the external surface; the external shield being configured to be electrically connected to the metallic wall via at least one metallic fastener used to mount the flange to the metallic wall; and
a generally tubular internal braided metallic shield so embedded in the body near the internal surface as to be coaxial with the external shield; the internal shield being configured to be electrically connected to the conductor; the internal shield being longer than the external shield and being provided with an outwardly flaring distal end.
Other objects, advantages and features of the present invention will become more apparent upon reading of the following non restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings.